This invention is concerned with digitizing the surface of an irregularly shaped article, e.g. a shoe last.
There is described in U.S. Pat. No. 4,817,222 one method of digitizing the surface of an irregularly shaped article, e.g. a shoe last, using an apparatus which comprising a support for supporting an article the surface of which is to be digitizing, a digitizing assembly comprises a light source by which a light beam can be directed along a path so as to strike the surface of such article, and means for detecting the point of incidence of the light beam with said surface, means for effecting relative rotational movement between the support and the digitizing assembly about an axis of rotation which passes lengthwise through an article supported by the support and in the direction of which the path of the light beam extends perpendicularly thereto, and also for effecting relative lengthwise movement therebetween in a direction along said axis, means for monitoring the relative rotational position of the support and digitizing assembly and also the relative lengthwise position thereof at regular intervals, means for supplying signals corresponding to each such monitored position and also for supplying a signal having a value corresponding to the distance of the point of incidence of the light beam from the axis of rotation at each such monitored position, whereby a succession of coordinate signals representing point positions, i.e. points of incidence in terms of relative rotational and lengthwise movements as well as distance, is derived, and means for converting such succession of signals to provide a set of data by which the surface of the article is defined.
In said apparatus the digitizing assembly is in the form of a so-called laser range sensor, utilising the principle of triangulation, which sensor is fixed in relation to the rotational movement of the shoe last but is movable linearly along an axis parallel to the axis of rotation of the last, and by which a beam of light is directed towards the shoe last and the point of incidence of said beam with the shoe last surface is detected. Using such an arrangement, whereas for the most part the surface of the last can be adequately digitized, problems do arise where, for example, the point of incidence of the light beam with the shoe last surface cannot be "seen" by the detector; this occurs where, for example, the portion of the last surface upon which the light falls is cut away or is generally hidden by a more bulbous portion of the shoe last, such as can arise where the light is incident upon the side portions of the crown of the last. Thus, depending upon the particular shape of the shoe last it is not always possible, using a fixed sensor arrangement, to monitor each and every point of incidence of the light with the last surface.
In order to avoid this problem of the sensor being "unsighted", it is proposed in WO-87/01194 to utilize an optical system including two mirrors by which each point of incidence of the light beam with the article surface to be digitized is looked at by both mirrors and the images thus received by the mirrors are superimposed upon one another and thus supplied to the sensor; in this way, if one of the mirrors is "unsighted", nevertheless the composite, i.e. overlapped, image is nevertheless complete. In this system, however, it is a question of using a line of light, rather than a point, and scanning the whole length of the line in one scanning operation. Moreover, the image from one of the mirrors must be reversed, using a further mirror which allows the image from the other mirror to pass therethrough and all three mirrors must be accurately maintained in alignment with one another and with the sensor in order to ensure that the two images are coincident, and furthermore each mirror must be so arranged that the angle subtended between the light beam directed from the light source towards the article surface and the light beam directed from the light source towards the article surface and the image directed towards the mirror is the same in the case of each of the two mirrors; otherwise the images from the two mirrors will not be coincident.